This invention relates to a method of detecting the air/fuel ratio of an air-fuel mixture subjected to combustion in a combustor, such as a combustion chamber of an internal combustion engine, by using an oxygen sensor of a specific type disposed in the combustion gas exhausted from the combustor.
In the field of internal combustion engines, particularly in automotive engines, it has been popularized to detect changes in the air/fuel ratio of an air-fuel mixture actually supplied to the engine as the basis for feedback control of the air/fuel ratio by detecting changes in the concentration of oxygen in the exhaust gas of the engine, since it is more practical to provide an oxygen sensor to the exhaust system of the engine than to the intake system. An oxygen sensor prevailing for this purpose is of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side. This oxygen sensor is used with the maintenance of a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer, so that the sensor generates an electromotive force, the magnitude of which varies according to a difference between an oxygen partial pressure in a sample gas to which is exposed the measurement electrode layer and the reference oxygen partial pressure. When used in an engine exhaust gas, this oxygen sensor exhibits a great and sharp change in its output on the occurrence of a change in the air/fuel ratio across a stoichiometric air/fuel ratio. Accordingly this oxygen sensor is quite suitable in the case of an engine being operated with a stoichiometric or a nearly stoichiometric air-fuel mixture.
In automotive gasoline engines, the development of so-called lean-burn-engines has been in progress with the view of improving the combustion efficiency and as a consequence further advancing the exhaust emission control by the employment of a considerably lean air-fuel mixture, i.e. a mixture having an air/fuel ratio considerably above the stoichiometric air/fuel ratio 14.7 (by weight).
In lean-burn-engines, however, the control of the air/fuel ratio encounters a problem that it is impossible to accurately detect or estimate an actual air/fuel ratio of a lean mixture by the installation of an oxygen sensor of the above described type in the exhaust system since there occurs little change in the magnitude of the electromotive force of the sensor when the air/fuel ratio changes only on one side of the stoichiometric ratio. Therefore, conventional lean-burn-engines are compelled to employ open-loop air/fuel ratio control systems which require the addition of various elements to the engine intake system. However, these control systems are disadvantageous not only in their complicatedness and expensiveness but also in their unsatisfactory accuracy and responsiveness.
A method of detecting a lean air/fuel ratio is proposed in SAE Paper (Society of Automotive Engineers, U.S.A.) No. 78.0212 (1978). In this method, use is made of an oxygen sensor having a tubular layer of zirconia (a typical example of oxygen ion conductive solid electrolytes) which is coated with reference and measurement electrode layers respectively on the inside and on the outside with compensation for temperature and using air as the source of the reference oxygen partial pressure.
However, the magnitude of changes in an electromotive force generated by the oxygen sensor disposed in the exhaust gas and used in this manner is very small insofar as the air/fuel ratio varies within a lean range, so that it is not easy to accomplish feedback control of the air/fuel ratio based on the output of the oxygen sensor. For example, an oxygen partial pressure P in the exhaust gas is about 1.times.10.sup.-2 atm when the air/fuel ratio is about 15 and becomes about 4.times.10.sup.-2 atm when the air/fuel ratio varies to about 18, while the reference oxygen partial pressure (of air) P.sub.o is about 0.21 atm. According to the Nernst Equation, the magnitude of a change .DELTA.E in the electromotive force resulting from the change of the oxygen partial pressure P from P.sub.1 =1.times.10.sup.-2 to P.sub.2 =4.times.10.sup.-2 at a constant temperature T (.degree.K.) is expressed as follows: ##EQU1## where R is the gas constant and F is the Faraday constant. When the exhaust gas temperature T is 900.degree. K., ##EQU2## This value for .DELTA.E is too small to utilize as a feedback signal in a practical air/fuel ratio control system since various factors such as dispersion in electromotive force generating ability of industrially produced oxygen sensors, influence of the exhaust gas temperature on the electromotive force and errors in the measurement by the influence of noises must be taken into consideration.
According to SAE Paper No. 76.0312 (1976), it is possible to detect changes in air/fuel ratio of a lean mixture by utilizing a certain oxide semiconductor such as CoO, which exhibits a change in its resistivity in response to a change in an oxygen partial pressure in an environmental gas atmosphere, as the sensitive element of an oxygen sensor which is disposed in the exhaust gas. It is necessary, however, to maintain this sensitive element at a very high temperature such as about 900.degree. C. because at lower temperatures CoO tends to transform into more stable Co.sub.3 O.sub.4 of which the resistivity is not significantly influenced by an oxygen partial pressure in an eivironmental gas atmosphere.
Accordingly an oxygen sensor of this type needs to be provided with a heater. In practical applications, the provision of a heater which is continuously operated at a temperature as high as 900.degree. C. offers various problems such as deterioration of not only the heater material but also the CoO sensing element itself and complicatedness of an indispensable temperature compensation circuit. Because of these problems, this sensor is unsuitable for use in popular apparatus such as automobiles.
U.S. Pat. No. 3,941,673 shows a zirconia tube oxygen sensor which utilizes a noncatalytic material such as gold or silver as the material of its measurement electrode layer to be exposed to an engine exhaust gas and is of use for detection of air/fuel ratios of a rich mixture. From a practical viewpoint, however, this oxygen sensor is unsatisfactory in that the measurement electrode layer is insufficient in its endurance in a high temperature and high velocity fluid flow such as an exhaust gas in an automotive engine exhaust line because of a relatively low melting point and softness of the electrode material and in that the relationship between oxygen partial pressure in a sample gas and electromotive force of the sensor tends to fluctuate because of adsorption of the sample gas in the noncatalytic electrode layer. Besides, as a shortcoming common to conventional oxygen sensors comprising a zirconia tube large enough to serve as a structural member, a temperature compensation circuit for this oxygen sensor (to compensate for changes in the electromotive force with changes in temperature) becomes considerably more costly because of a large heat capacity of the zirconia tube.